Problem: On the first 3 tests of her language class, Ashley got an average score of 84. What does she need on the next test to have an overall average of 87?
Answer: Let her score on the next test be $x$ The sum of all of her scores is then $3 \cdot 84 + x$ The same sum must also be equal to $4 \cdot 87$ Solve: $x = 4 \cdot 87 - 3 \cdot 84 = 96$.